1. Field of the Invention
The invention is directed to the field of manufacturing electronic devices. More specifically, the invention is directed to cleaning systems and methods used to remove foreign material such as flash and other contaminants from external leads of the electronic devices.
2. Description of Related Art
Intricate electronic devices such as semiconductor integrated circuits (ICs) (or "chips") are typically housed in an encasing referred to as a "package." The package typically includes a "lead frame" that is electrically connected to the IC within the package, and extends outward to allow electrical connection of the IC to a circuit board or other product. One of the most popular package types used in the art is known as the "epoxy molding" or "plastic" package. With this type of package, the IC and lead frame are enclosed or encapsulated by a plastic or resin material that serves to protect the chip from moisture, contamination, and other physical or environmental conditions.
The basic process flow for the manufacture of a plastic package of a semiconductor device starts with the attachment and bonding of the IC die to a lead frame containing a number of leads. A preseal inspection often is performed to ensure that the die is attached correctly. A plasma cleaning step may be included prior to wire bonding to remove any residual photoresist, or other organic contaminants on the bonding surfaces of the die. The bonding surfaces of the IC die are then respectively connected to individual leads on the lead frame with very thin wires during the wire bonding step. The lead frames and attached dies are then transferred to a molding area.
In the molding area, the frames are placed on a mold mounted in a transfer molding machine. The molding machine in turn injects epoxy or other plastic encasing material into the mold around the die on the lead frames, thereby forming an individual package around each lead frame leaving only external ("outer") leads exposed to the environment. A plating step is often used to coat the external leads of the package with a metal finish so as to improve the lead solderability, resulting in a more reliable electrical connection of the package and the printed circuit. After the epoxy sets in the mold, the frames are removed and placed in an oven for final curing.
Often, as a result of the molding step, excess plastic, resin, wax or other organic residue material, such as trace oxides or contaminants, can be found around the casing of the encapsulated chip, as well as on and between the external leads of the chip. As shown in accompanying FIG. 1, a typical lead frame 10 is used to provide external electrical connections to IC die 20. Once the die 20 is mounted on the lead frame 10 and the appropriate wire bonds are made to inner leads 24, 28, the lead frame 10 is exposed to an encapsulation process step. In this process step, the die 20 and inner leads 24 (around the boundary indicated by the dashed line 26) will be encapsulated by a molded plastic casing 30 (FIG. 2).
During this encapsulation process, the lead frame 10 is inserted into a mold cavity while the leads 16 extend outside of the cavity. The mold is heated and the plastic is injected into the mold in liquid or semi-liquid form under very high pressure. Due to its fluidity, the plastic material leaks out of the mold through any crevices where the sealing is imperfect. As a result, excess encasing material 36 (FIGS. 2 and 3) "bleeds" out of the encapsulated chip package 30 onto and between leads 16. This excess encasing material 36 is referred to in the art as "flash." Flash is detrimental to the fabrication process in that its presence adversely affects the subsequent soldering, trimming and forming operations, in addition to the overall electrical characteristics of the device.
To avoid the problems caused by flash, another process step often referred to as "deflash" or "flash removal" is commonly added to the basic process flow. Most of the known methods employed to perform this deflash step involve exposing the device to chemical solvents or abrasive blasting. The flash removal system shown in U.S. Pat. No. 5,318,677, for example, performs the deflashing step by dipping the components in a bath of glycerol and phosphate. In another example, the cryogenic deflashing system of U.S. Pat. No. 5,676,588, attempts to remove flash by exposing devices to cryogenic material such as liquid nitrogen (at a temperature of about -60.degree. F. or below) and blasting the devices with particulate media. Many other variations of these two types of deflashing procedures are known in the art.
The chemical solvent-based deflashing procedures are problematic because of the liquid waste that is produced leading to environmental concerns regarding the handling and disposing of the used solvents.
The essential disadvantage of the abrasive-type of flash removal systems is that minute quantities of the blasting abrasive become embedded in the surface of the electronic part (e.g., lead). These embedded particles must be carefully removed before proceeding with other process steps such as plating the surface with a metallic (solderable) coating. The abrasive deflashing procedure is also often incomplete in regions leaving very thin layers of residue that are very difficult to detect upon inspection with the naked eye.